The present invention relates to differential amplifiers, especially error amplifiers, used in control and regulation systems, where it is necessary or useful to control, at the time power is applied, the rise of the voltage on the output node of the amplifier toward the supply voltage, through the charging of capacitances associated with the output node. This is important, e.g. in DC-to-DC converters, to avoid overvoltage of the regulated output voltage during the start-up phase, and in positioning motors.
In control and regulation systems, differential amplifiers (error amplifiers) are widely used which have an output stage composed of a pair of transistors functioning in a push-pull mode (for example a "pull-up" transistor connected between the output node and the supply rail and a "push-down" transistor connected between the output node and ground) and in which the output node is often connected directly to the input of a comparator. At start up, an out-of-balance condition of the inputs of the amplifier may cause a sudden saturation rise of the output node toward the supply voltage. A steep rising of the output voltage toward the supply may cause undesired over-shoot phenomena of the parameter value controlled by the regulating system.
Known solutions generally consist in slowing the voltage rise at the output node at start up, either by introducing a circuit device for controlling the voltage rise (slew rate) on an internal driving node of the amplifier or on the output node itself, for example by tying the output node voltage to the voltage of a so-called "soft-start" capacitance which is charged with a constant current.
In many applications, especially in case of error amplifiers, the introduction of such a device within the inner core circuit of the amplifier may present remarkable difficulties, since a few millivolts may be sufficient to completely unbalance the output stage of the amplifier. In these cases, control of the rise time of the voltage at the output node must necessarily be effected at the output node itself without altering the core of the amplifier.
A known solution is based on the use of a diode or a PNP transistor with an emitter connected to the output node, a collector connected to a common ground node of the amplifier and a base connected to the armature of an external "soft-start" capacitor. The external soft-start capacitor may be charged through a charge path that is functionally external to the structure of the amplifier. The PNP transistor prevents a too steep rise of the voltage on the output node of the amplifier.
This known solution has several drawbacks and limitations. There is in fact a constant tendency to reduce as much as possible the size of the external components, such as the soft-start capacitor, as well as of minimizing current absorption. The use of a diode or a PNP transistor, intrinsically capable of producing a "base" current in the order of milliamperes, may often be incompatible with a soft-start capacitor requiring a charge current of few milliamperes. In other words, the base current of the transistor may represent per se an excessive contribution to the charge current of a soft-start capacitor, which for other reasons should be as small as possible.
To solve these difficulties and drawbacks, the present application advantageously provides a control circuit capable of tying the output voltage of an amplifier, by employing an external soft-start capacitor of small size while keeping the total charge current of the external soft-start capacitance, in the order of microampers.
Basically, the circuit of the invention implements a feedback that controls the driving level of the pull-up transistor in the amplifier's output stage, which transistor outputs the charge current of the soft-start capacitor, in order to limit the charge current delivered through a diode to the external capacitor connected to the dedicated "soft-start" pin of the integrated device, thus implementing a gradual rise of the output voltage.
The feedback loop consists essentially of a complementary pair of current mirrors, through which the charge current that flows through the charge diode to the external soft-start capacitor is "mirrored", in a suitable ratio, on the base of the same transistor of the push-pull output stage of the amplifier that delivers the charge current, in order to partially reduce its drive conditions.
Within the control loop, a certain current balance is achieved such that the current forced by the respective bias current generator of the output transistor that delivers the charge current of the output node, is counterbalanced, at least partially, by a current that is a function of the effective charge current, and which is fed to the base of the transistor through the current mirror of the feedback circuit of the invention.
Therefore, the output node gradually charges itself toward the supply voltage.
An auxiliary current generator may be employed for ensuring a complete charge of the external soft-start capacitor when, at start up, the output node of the amplifier stabilizes itself at a certain level imposed by the regulating chain of which the amplifier is part.
In any case, upon the reaching of a fully charged state by the external soft-start capacitor, the charge diode (which is part of the first current mirror) cuts itself off, thus isolating in practice the control soft start circuit from the functional circuit of the amplifier.